This invention relates generally to the field of telephone protector modules of the type used in conjunction with individual telephone subscriber circuits to protect the same against the deleterious effects of excessive voltage and corresponding amperage, which, if not grounded, can extensively damage the telephone system.
Such devices are well-known in the art, and almost all include a component having a pair of carbon electrodes which are placed in mutually spaced relation to define an air gap. One electrode is connected to a source of ground potential, and the other electrode is connected to an individual subscriber line. When excessive voltages are placed upon the line, the current temporarily arcs over the gap to ground potential. When the condition causing the excessive current terminates, the gap provides a means for insulating normal currents from ground potential and prevents current leakage. Such devices, sometimes referred to as spark gap arrestors, have proven effective within a relatively limited life span. Rural electrification authority standards existing for some time have required a useful average life of 40 firings before failure. Most conventional carbon electrodes do not achieve this goal, despite considerable research involving such factors as the quality of carbon from which the electrodes are formed, the type of binding material employed during manufacture, and subsequent treatment to reduce the tendency to disintegrate.
It has been established that electrode failure is attributed to two major factors, one being the presence of excessive humidity between the electrode surfaces during firing. This problem has been somewhat alleviated by the post-fabricating step of dipping the electrodes in dilute solution of propylene glycol or other dilute sealants to cause a thin film of a wax-like coating to at least partially seal the exposed and internal surfaces of the carbon particles comprising the electrode.
More serious is the gradual degradation of the electrode surface caused by upheaval and fission of the electrode surfaces occuring during the generation of heat incident to arcing.
Some progress has been made in ameliorating this problem. As disclosed in U.S. Pat. No. 3,703,665; granted Nov. 21, 1972 to Robert A. Yerance, et al, it has been found that the creation of raised plateaus separated by grooves on the operative face of at least one electrode, permits the venting of the air gap allowing the discharge of carbon particles which have been loosened under erosion caused by arcing when the device is in operation. The heat generated expands air disposed within the gap, the air blowing the loosened particles through the passageways formed by the grooves. This teaching also refers to specific depths of grooves in relation to gap distance between the electrodes for the purpose of eliminating, or at least inhibiting, the formation of moisture in the gap during periods of changing atmospheric pressure and temperature. The use of particular grooved patterns, stated as a ratio of the area of plateau to combined plateau and grooved area, is employed to reduce surface eruption and cratering caused by repeated firings.
While such electrodes offer a material advantage in terms of longevity as compared to conventional flat-surfaced electrodes, the grid-like patterns employed do not provide the best venting of loose particulate material. Additionally, the molding of relatively complex patterns on the exposed operative surfaces of the electrode is a difficult manufacturing operation, and breakage rates are correspondingly high. The venting of such patterns is usually not along straight lines, and maximum possible venting action is therefore not obtained.